bigbulls it's all about energy absorption and disappation. The energy released when the bullet is fired is the same regardless whether the gun is fired or not. But when the gun is held stationary, the energy isn't disappated instead it is distributed throughout the gun and the weakest link will lose. The weakest link in any scope is the reticle assembly.

Imagine rining a bell. If you whack it good while it is held firm you run the risk of splitting the bell. If it is free to move the energy from the blow is distributed. Same way with a large truck hitting a wall versus a movable object, the more energy that can be absorbed and shed through movement, the less violent the reaction too acceleration/deceleration.

I can promise you that EVERY single year I will get customers who bring scopes back with problems that are DIRECTLY related too the gun having been fired while affixed too an immovable object.

It's not really a matter of pure physics, more an example of the limits of a scopes components,
RA